As to research on resistance reduction for vessels, a lot of results on wave-making resistance have been acquired. However, a distinct result on frictional resistance, which is said to be responsible for about 80% of the entire resistance of a large-scaled vessel, has not been acquired up to the present, because complicated turbulence phenomena are related thereto. When a vessel is sailing, a turbulent boundary layer is developed on or below the waterline along the surface of the hull. Accordingly, a frictional resistance is exerted on the surface of the hull, thereby reducing the propulsive performance of the vessel. In recent years, as to means for reducing such a frictional resistance on the surface of the hull, a microbubble system, which reduces the resistance by injecting microbubbles into the boundary layer along the surface, has received attention as a promising measure for reducing a frictional resistance and been researched. It is considered that the micro bubble system is a promising frictional resistance reduction device in an era from now on when energy saving is desired, because the system is remarkable in frictional resistance reduction effect, relatively resistant to fouling, has no problem of environmental contamination of the ocean and the hydrosphere.
However, this micro bubble system requires power because the system generates bubbles with a diameter of a several hundred μm or less from underwater sides of the vessel and the bottom of the vessel overcoming the water pressure. Considerations are required that increase a net reduction effect in consideration of both of a frictional resistance reduction effect and power for generating bubbles. Further, there is a problem how the frictional resistance is effectively reduced in a vessel sailing on the ocean and rivers, which receives various disturbances, such as waves and streams, and receives influences owing to freight conditions and navigational conditions. In particular, buoyancy acting on bubbles is small on microbubbles with a diameter of several hundred μm or less. When the micro bubbles are injected to the bottom of the vessel, the micro bubble is susceptive to influences of disturbances and easily diffused. Moreover, previous researches have mainly been of a laboratory level and researches using a model ship, and have not considered the disturbances and the navigational conditions.
Further, it is required, as a precondition, that construction and installation of such a gas supply or charged air device does not cost enormously.
In view of the present states of researches, the inventors of the present application use an actual vessel, equip the hull thereof with a gas inject device capable of injecting bubbles with a diameter larger or equal to that of micro bubbles, investigate a relationship between the inject of bubbles and a frictional resistance reduction effect in situations analogous to actualities, such as vessel conditions, navigational conditions, oceanographic conditions, search for increase in net reduction effect, and acquire a lot of findings. Further, researches are pursued for optimizing the particle diameters of bubbles in order not to diffuse the gas injected from the gas inject device in the sea and separate the gas from the bottom of the vessel, and also on components, devices and the shape of the bottom of a vessel. Moreover, the researches using such an actual vessel have determined that the injected bubbles are convolved into propeller means, reduce the propeller efficiency and adversely affect a net frictional resistance reduction effect.
On the other hand, marine diesel engines as the main engines of large scale vessels are equipped with turbochargers for compressing and providing air for combusting fuel. In recent years, the marine diesel engine has realized high power and high efficiency. This is because, in a large measure, the efficiency of the turbocharger has been increased.
Recently, the efficiency of the turbocharger has become higher. There appears a turbocharger exerting efficiency equal to or higher than that required by the engine. This means that air required for the engine or exhaust gas generated by combustion of fuel is more than necessary.
Energy-saving techniques that drive exhaust gas power turbines using the residual exhaust gas and rotate electric generators have generally become widespread (e.g., turbo compound system). There also arises a concept utilizing the residual exhaust gas as a source of generating the bubbles.
For example, as shown in following Patent Documents 1 to 4, concepts that use the exhaust gas from turbochargers and generate bubbles have been disclosed.
Patent Document 1 discloses a technical idea that adjusts fuel adjustment means and gas drawing rate adjustment means to reduce fuel consumption while drawing exhaust gas from a turbocharger and injecting the gas into water to reduce frictional resistance, and adjusts the flow rate of the exhaust gas so as to minimize the amount of fuel supply as a control. However, the idea disclosed in Patent Document 1 only controls the drawing rate so as to reduce the amount of fuel supply to a main engine with respect to a prescribed vessel velocity. The optimal control is not necessarily realized. Further, since only the exhaust gas is used, it is not used in a case of affecting the environment.
Patent Document 2 discloses a technical idea that provides a drawing port at a low pressure spot of a turbocharger compressing gas for a main engine and releases the drawn gas into water. Although the idea disclosed in Patent Document 2 refers to a valve for adjusting the flow rate, the idea discloses no specific control method.
Patent Document 3 discloses a technical idea that provides a branch line for a compressed air line of a turbocharger provided at a main engine, draws scavenged gas from a spot downstream from the intercooler and discharges bubbles. However, this idea discloses no specific control method either, and has a possibility of adversely affecting the operation of the main engine.
Patent Document 4 discloses a technical idea of a turbo compound system (exhaust gas passes through the blower turbine and the blower blows the gas to discharge bubbles) that branches exhaust gas out of a main engine, provides a turbine at a branch line, causes the turbine to drive a blower to discharge bubbles from the hull. However, the idea discloses no specific control method, and has a possibility of adversely affecting the operation of the main engine.
Patent Document 5 provides the hull with an air injection unit injecting air from a nozzle via an air injection control unit, provides a sensor detecting air bubbles or an air sheet released therefrom at the bottom of the hull, inputs signals from the sensor into the hull operation measurement unit, inputs wave information, relative wave information, displacement, movement velocity, acceleration, a direction, detected conditions of bubbles and a wind velocity and a wind direction into the hull operation measurement unit, detects bubble conditions on the basis of bottom of the vessel pressure information, bubble distribution information, frictional force information and hull resistance information, and optimally controls the released air layer and the bubble conditions.
Since such a configuration is adopted, various pieces of detected information are limited. The control is not performed by determination of navigational situations, such as the relative velocity between the vessel and the water and the draft conditions, and vessel situations, such as operating conditions of the vessel and the engine conditions. This cannot appropriately control the air bubbles and the air sheet. Further, this does not control the number of air injection units according to various pieces of detection information.
Patent Documents 6 and 7 disclose ideas that control supply of pressurized gas and exhaust gas according to pressure and the draft, supply the gas when the pressure rises to a certain degree and exceeds a pressure determined by a depth of water of protruding (injecting) and stop the gas when the pressure decreases. However, these documents do not disclose or suggest an idea that supplies gas at a higher level, stops even at a higher level or blocks after a backflow occurs.
Patent Document 8 discloses a technical idea that pertains to a micro bubble injecting device, integrally forms a fluid guide plate, which guides bubble-water mixture fluid aft, at the front end of a fluid nozzle provided at the hull, and converts a kinetic energy when the bubble-water mixture fluid is injected into propulsion of the vessel. However, the idea disclosed in Patent Document 8 is incapable of preliminarily determining the direction of the fluid to be injected. Further, there is a possibility that the fluid guide plate itself becomes a resistance preventing the vessel from sailing.
Patent Document 9 discloses a technical idea that injects bubbles from an air duct, provides strengthening devices straitening a drift at an upstream and downstream sides of the air duct, and secures a friction reduction region by spreading the bubbles along the drift. However, according to the idea disclosed in Patent Document 9, the strengthening device only plays a role that straitens the drift so as to smoothly flow from the upstream side to the downstream side with respect to the air duct. Therefore, when the velocity of the currents is high, there is a possibility that the bubbles are diffused on injecting. Further, since the air duct protrudes, there is a possibility that the air duct becomes a resistance preventing the vessel from sailing. In a case where the vessel is carried, when the vessel is mounted on a supporting stage, such as blocks, the air duct becomes an obstacle in security and working efficiency.
Patent Document 10 discloses a technical idea that provides the bottom of the vessel with a regulating member for regulating seawater streamed to the side of the bottom of the vessel by inject stream generation means and air injected by air injecting means so as not to escape toward the sides of the hull. However, the idea disclosed in Patent Document 10 provides the regulating member at the sides of the vessel and the regulating member is fixed such that a part thereof protrudes from the bottom of the vessel. Therefore, when the vessel is carried into a dock because of maintenance of the vessel, if the regulating member hits a supporting stage such as blocks, the vessel cannot only be fixed in a stable manner, but also the regulating member becomes an obstacle, thereby reducing working efficiency. Further, the regulating member itself is provided at a side of the hull. Accordingly, the contact area with seawater becomes significantly large, and the regulating member becomes the resistance adversely affecting the propulsion of the vessel, thereby reducing the net frictional resistance reduction effect of the air. Further, since the configuration where the air is injected from a center portion in the longitudinal direction of the hull is adopted, the frictional resistance reduction effect is limited.
Patent Document 11 discloses a technical idea where, in a vessel having a structure of blowing air into the bottom of the vessel, the bottom of the vessel has a substantially planar structure with respect to a flow, a lot of grooves are formed on the bottom of the vessel so as to be structures for taking air into the grooves and air is introduced, thereby separating the bottom of the vessel and water from each other. However, the idea disclosed in Patent Document 11 has a structure where the air is spread at the bottom of the vessel and the bottom of the vessel is covered with an air layer. The bottom of the vessel is accompanied by the air, but does not have a member and the like holding the air. An inclination of the vessel owing to disturbance of the wave, the flow and the like diffuses the air toward an area with lower pressure. This reduces the amount of air accompanying the bottom of the vessel. It is inevitable that the frictional resistance reduction effect is reduced accordingly.
On the other hand, for example, measures as disclosed in Patent Documents 12, 13 and 14 have been taken in order to reduce the frictional resistance of the vessel using micro bubbles.
This Patent Document 12 attaches an air injecting device communicated with the blower to a board side of the bow of the hull, attaches an air injecting device communicated with a compressor to the bottom of the vessel at the bow of the hull, covers areas around the both sides in the board direction of the bottom of the vessel and the sides of the vessel with micro bubbles generated by the air injecting device of the blower, covers areas around middle section in the board direction of the bottom of the vessel with micro bubbles generated by the air injecting device of the compressor, and entirely covers the wet surface of the hull.
Since such a configuration is adopted, only one air nozzle is provided on each surface of the sides of the vessel or the bottom of the vessel. It is incapable of controlling the manner of blowing bubbles according to the navigational condition of the vessel. The reduction in frictional resistance is limited. Further, in a typical blower, the gas supply or charged air rate varies and the air blowing rate varies according to variation in pressure owing to vertical movement of the waves. This does not causes stabilized reduction in frictional resistance. Further, the compressor has a limitation on a rate of air to be blown by the compressor.
Patent Document 13 provides a plurality of openings at upper and lower positions with different static pressures of outer plates of the hull, attaches air injecting devices to the respective openings, provides a gas drawing type blower device including a blower having a low discharge pressure and blower having a high discharge pressure on the deck at the bow, communicates the air injecting device on the upper stage and the blower with the low discharge pressure by the air supply pipe, and communicates the air injecting device on the lower stage and the blower with large discharge pressure by the other air supply pipe.
Since such a configuration is adopted and, in particular, one motor drives two blowers, it is incapable of controlling the manner of blowing bubbles according to the navigational condition of the vessel. The reduction in frictional resistance is limited. Further, the nozzles are arranged the upper and lower stages of the vessel. Accordingly, in particular at the nozzle on the upper stage, generation of the bubbles is susceptible to vertical movement of the waves. In the blower, the gas supply or charged air rate varies and gas supply or charged air rate varies according to variation in pressure owing to vertical movement of waves. Therefore, this idea is not stabilized and effective measures for reducing the frictional resistance.
Patent Document 14 provides a plurality of nozzles in a limited region in the proximity of the bow in an aggregated fashion in order to reduce frictional resistance between the hull and water by injecting gas into water from proximity of the bow during sailing and thereby causing micro bubbles to intervene at the outer shell of the hull. Hydrostatic pressures generated in the proximity of the nozzles diffuse the bubbles over the bottom of the vessel.
Since such a configuration is adopted, the bubbles from the nozzles on a curved surface below the valve in the bow section are susceptive to the navigational conditions of the vessel. Since the idea is a passive bubble diffusion system, the method is incapable of positively controlling generation of the bubble according to the navigational condition of the vessel and thereby effectively reducing the frictional resistance. Further, since the nozzles are arranged on an arc toward the downstream side in the limited region in the bow section, the resistance at the bow section cannot effectively be reduced.
Patent Document 1
Japanese Patent Laid-Open No. 2001-097276
Patent Document 2
Japanese Patent Laid-Open No. 2001-48082
Patent Document 3
Japanese Patent Laid-Open No. 11-348870
Patent Document 4
Japanese Patent Laid-Open No. 11-348869
Patent Document 5
Japanese Patent Laid-Open No. 2004-188993
Patent Document 6
International Publication No. WO2005122676
Patent Document 7
International Publication No. WO1999028180
Patent Document 8
Japanese Patent Laid-Open No. 8-243368
Patent Document 9
Japanese Patent Laid-Open No. 2003-160091
Patent Document 10
Japanese Patent Laid-Open No. 5-116672
Patent Document 11
Japanese Patent Laid-Open No. 6-191396
Patent Document 12
Japanese Patent Laid-Open No. 10-24891
Patent Document 13
Japanese Patent Laid-Open No. 10-100983
Patent Document 14
Japanese Patent Laid-Open No. 2000-296795